Field of the Invention
The invention relates to a circuit configuration for producing a quadrature-amplitude-modulated transmission signal, which contains a coder to an input side of which a digital user signal can be supplied and which, on an output side has a respective connection for a real part and for an imaginary part of a quadrature-amplitude-modulated signal that is to be transmitted. The circuit further has a device for digital conversion of the signal to be transmitted to a radio-frequency band, with a connection for the real part and the imaginary part, and a connection for a transmission system, which contains samples derived at an output frequency.
Such a circuit configuration for quadrature-amplitude-modulated (QAM) signals is disclosed in Published, Non-Prosecuted German Patent Application DE 199 39 588 A. The transmission configuration is, for example, used in a cable modem in order to send signals in a back channel from the terminal via the broadband cable network. A QAM transmitter is required to be able to produce QAM-modulated signals and carrier frequencies in the range from about 5 MHz to 65 MHz. Owing to the required carrier frequency accuracy, direct digital conversion to the final frequency band without any prior analog conversion, as is carried out by the known QAM transmitter, is not feasible. The transmitter contains a coder, which produces samples for the real part and the imaginary part of the signal to be transmitted in the baseband signal. Following subsequent low-pass filtering of the signal components, the conversion to the carrier frequency is carried out by a Cordic calculation unit (Cordic).
The maximum carrier frequency for a QAM transmission signal is 65 MHz. In order to satisfy the sampling theorem, a clock frequency of about 200 MHz is required in practice. In the circuit configuration described in Non-Prosecuted, German Patent Application DE 199 39 588 A, the low-pass filter and the Cordic are thus operated at 200 MHz. The circuit complexity for implementation is correspondingly complicated.
U.S. Pat. No. 5,412,352 discloses a circuit configuration for broadband RF transmission, by which direct digital conversion to the radio-frequency band is possible. The circuit contains a digital mixer and a variable oscillator, which contains a digital sine and cosine generator. The digital filtering is carried out in two stages. The first stage contains a non-recursive filter, and the second stage contains a specific interpolation filter. This results in the production of a narrowband-filtered signal.
It is accordingly an object of the invention to provide a circuit configuration for producing a quadrature-amplitude-modulated transmission signal that overcomes the abovementioned disadvantages of the prior art devices of this general type, which can be produced with relatively little circuit complexity.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for producing a quadrature-amplitude-modulated transmission signal. The circuit configuration contains a coder having an input for receiving a digital user signal and outputs for outputting a real part and an imaginary part of a quadrature-amplitude-modulated (QAM) signal to be transmitted. A device for digital conversion of the QAM signal to be transmitted to a radio-frequency band, is provided. The device is connected to the coder and has a first input for receiving the real part and a second input for receiving the imaginary part of the QAM signal. The device has a first output for outputting the real part and a second output for outputting the imaginary part. A first interpolation filter is connected to the first output of the device for receiving the real part, a second interpolation filter is connected to the first output of the device for receiving the real part, and a third interpolation filter is provided for receiving the imaginary part. A multiplexer is connected to the second interpolation filter and to the third interpolation filter to switch between the second interpolation filter and the third interpolation filter at an output frequency. A terminal is connected to the multiplexer and at which the QAM signal may be tapped off, the QAM signal contains sample values sampled at the output frequency. A switching device is connected to the device, the first interpolation filter, the second interpolation filter and the third interpolation filter. In a first setting of the switching device the first interpolation filter is connected to the first output for the real part of the device, and in a second setting of the switching device the second interpolation filter is connected to the first output for the real part of the device and the third interpolation filter is connected to the second output for the imaginary part of the device. The switching device is controllable in dependence on the output frequency.
By use of additional circuit complexity in the form of interpolation filters, a multiplexer and a switching device, the invention results in the majority of the circuit running at half the output clock frequency fa of 200 MHz. Only the multiplexer on the output side is operated at the clock frequency of 200 MHz, which is required in practice on the basis of the sampling theorem. The circuits can thus be configured using conventional circuit techniques. The complexity required by virtue of the circuit parts that are also required according to the invention is thus more than compensated for.
In the circuit configuration according to the invention, a distinction is drawn between two bands for the carrier frequency that is to be produced. There is a lower frequency band between 5 MHz and a frequency f1, and a higher frequency band between f1 and 65 MHz. The frequency f1 occurs at about 20 MHz to 30 MHz. The frequency f1 is expediently chosen to be fa/8, where fa is the sampling frequency of the transmitted signal. Different signal paths are provided for the low frequency band and the high frequency band. Components can be used jointly, if the filters are partitioned appropriately. The QAM transmitter then contain two interpolation filter parts, which are connected downstream from the output for the real part and the imaginary part of the Cordic, and between which, on the output side, switching is carried out by the multiplexer and the sampling frequency of 200 MHz, with switching on the input side being dependent on the selected carrier frequency band.
In accordance with an added feature of the invention, the first interpolation filter has a first filter part and a second filter part connected in parallel on an input side and are each coupled on an output side to the multiplexer. The first filter part of the first interpolation filter and the second interpolation filter have an identical configuration. The second filter part of the first interpolation filter and the third interpolation filter have an identical configuration.
In accordance with an added feature of the invention, the first filter part is connected to the first output for the real part of the device. The switching device has a first changeover switch, by which the second filter part is connected, in the first setting, to the first output for the real part of the device and, in the second setting, is connected to the second output for the imaginary part of the device for digital conversion.
In accordance with an additional feature of the invention, a first mathematical sign inverting device for changing a mathematical sign is connected between the second interpolation filter and the multiplexer. A second mathematical sign inverting device for changing a mathematical sign is connected between the third interpolation filter and the multiplexer.
In accordance with another feature of the invention, the first filter part has an output and the second filter part has an output. The switching device has a second changeover switch that, in the first setting, is connected to the output of the first filter part and, in the second setting, is connected to the first mathematical sign inverting devices. The switching device also has a third changeover switch which, in the first setting, is connected to the output of the second filter part and, in the second setting, is connected to the second mathematical sign inverting device.
In accordance with a further feature of the invention, the multiplexer has inputs, and the second changeover switch and the third changeover switch are connected to the inputs of the multiplexer.
In accordance with a further added feature of the invention, there are provided low-pass filters, one of the low-pass filters is connected downstream from each of the outputs of the coder.
In accordance with a further additional feature of the invention, the device for digital conversion is a Cordic.
In accordance with another further feature of the invention, the first filter part, the second filter part, the low-pass filters and the Cordic process samples at a reduced frequency which is less than the output frequency, and in that the reduced frequency is half the output frequency.
In accordance with a concomitant feature of the invention, the switching device is controlled by the output frequency in such a manner that the first setting is selected when the output frequency is low, and the second setting is selected when the output frequency is higher than this.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for producing a quadrature-amplitude-modulated transmission signal, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. Mutually corresponding components are provided with the same reference symbols.